1. Field of the Invention
This invention relates to coded apertures for nuclear medicine imaging in general, and in particular to coded apertures providing graded attenuation.
2. Description of the Prior Art
Coded-apertures are used with radiation detectors, as an alternative to collimators, as medical diagnostic tools for detecting the radioactivity of an object under investigation, such as for determining the distribution of a radioactive isotope absorbed by a human body organ. An example of a radiation detecting device to which the present invention finds application is the Anger-type scintillation camera, the basic principles of which are explained in U.S. Pat. No. 3,011,057.
Collimators are used with a radiation detecting device to define radiation transmitting channels between the source of radiation (e.g. a human body organ) and a transducer (e.g. a scintillation crystal) so that radiation emitted from spatial areas of the radiation source can be mapped to corresponding spatial areas of the transducer. Collimators comprise radiation transmitting apertures or channels separated from each other by radiation opaque walls or septa. They ensure that only those photons travelling parallel to the channels will be detected by the transducer and that other photons are rejected. In a collimator imaging system, the detector is used to obtain a two-dimensional presentation of the three-dimensional distribution of the radiation source. Collimator imaging systems may be adopted for emission computerized tomography (ECT) to detect the source distribution from a plurality of viewing positions as the detector head is caused to precess about the patient. (See, for example, the disclosure of the commonly-owned, copending patent application Ser. No. 273,446 of Haas, et al., filed June 15, 1981, now U.S. Pat. No. 4,417,143, entitled "Improved Apparatus for Driving a Radiation Detector".) A display is produced showing the radioactive distribution in the object of study along a number of parallel section imaging planes.
Coded-aperture imaging is used as an alternative to collimator imaging to obtain information about the distribution of a radiation source. In coded-aperture imaging, a coded aperture called an "attenuation zone plate" is placed between the radiation source and the transducer of the radiation detector to modulate rays emanating from each point on the source, without collimation, to form an encoded record of the three-dimensional spatial distribution of the source. The attenuation zone plate comprises a radiation attenuating coded-aperture pattern such as a pattern of holes formed in a thin lead sheet. The radiation emitted in all directions from a single point on the radiation source passes through the coded-aperture pattern and produces a modulated image on the detector transducer--the distributions from all points being superimposed and representing an encoded record of the three-dimensional spatial distribution of the radiation source.
As described in J. Dowdey, et al., "Coded Apertures for Nuclear Medicine Imaging", Applied Radiology/NM, July-August 1977, pages 145-169, the coded-aperture imaging process is a two-step procedure. The first step is the production of an encoded record on a detection medium through modulation or "shadow casting" of the radiation source using a coded aperture. The second step is the decoding of the encoded record to provide usable source distribution information. The Dowdey, et al. article illustrates the process by an example coded aperture consisting of a pattern of holes in a thin lead sheet which is placed parallel to the face of the detector, between the detector and the radiation source. Each point on the radiation source casts a shadow that is of the same size and shape, but which is centered at a different position on the detector. Points located at different distances from the aperture cast different-sized shadows. The encoded record is a composite of the shadows of the aperture cast by all points in the source distribution. Decoding of the encoded record produces a usable image (typically presented as a series of tomographic slices) of the three-dimensional distribution of the radiation source.
The specific configuration of the coded aperture depends on the specific application and the decoding system to be used. One type of attenuation zone plate (e.g. the thin lead sheet formed with a hole pattern discussed in the Dowdey, et al. article) has a radiation transmission function T(x,y) having a value of one of the transparent points (the holes) and a value of zero at all other points. There are, however, graded attenuation zone plates (such as wedge-shaped plates or plates whose thickness varies sinusoidally) in which the transmission function T(x,y) of the pattern varies in a specified manner between the values of zero (radiation opaque) to one (completely radiation transparent).
In order to develop precise information about the three-dimensional spatial configuration of a radiation source, great precision is required in the layout and manufacture of attenuation zone plates. The required precision can present formidable problems in manufacture, especially in the manufacture of graded attenuation zone plates. Conventional attenuation zone plates are made of lead, with graded attenuation when present being achieved through careful machining of the lead to different thicknesses. To achieve varying radiation transmissivity between zero and one, portions of the plate must be formed of lead which is extremely thin and gradations are difficult to achieve with high precision. Furthermore, conventional graded attenuation zone plates are not adjustable to selectively vary the characteristics of the radiation attenuating pattern characteristics of the plate. Production of adjustable plates made of lead is difficult to achieve.